Electric wave system



Patented Aug. 3?, i9

ran

Verona, and Renato ill.

Fracassl,

N. 5., asslgnors to Bell Telephone Laboratories, incorporated, New York,N. $1., a corporation of New York Application septemher 9, 1941, Se No.41%,130

9 Claims. (on. 174) pair or telephone conductors or wires of theso-callecl open wire or of the cable type.

A feature of the invention comprises inserting,

at optimum spacings in the line, impedance net works or units thatenable the desired compen sation.

Another feature comprises inserting a plurality of attenuation andimpedance variation compensating units in each repeater section of alone transmission line, one unit being located centrally and other unitsbeing located nearer each end oi the repeater section than to the centerthereof.

Still another feature comprises compensating for the attenuationvariation with temperature of a transmission line by the provision of anappropriately proportioned compensator unit at about the center of a,repeater section oil the line, and compensating for attenuation andimpedance variation with temperature by the provision of anappropriately proportioned compensator unit near each end of therepeater section.

An ordinary telephone pair or line has a positive variation inresistance with temperature over the limited range embraced byatmospheric tem= peratures. The inductance, capacitance and leakance arealso afiected by change in temperature, but to a much smaller degreethan the resistance and attenuation. The impedance and variation inimpedance are more dependent on frequency but are affected bytemperature variation. In accordance with this invention, a transmissionline subjected to variation in temperature over the range of atmospherictemperatures has compensator networks or units inserted therein atspaced intervals. In one embodiment, three units per repeater sectionmay be provided in a nonloaded telephone pair, one substantially at thecenter of the section to compensate primarily for attenuation variationwith temperature, and one near each end of the section to compensate forboth attenuation and impedance variation. The compensator unit maycomprise a temperaturedependent resistance or thermistor having anegative temperature coeficient of resistance, and a resistance of zeroor of low temperature coemcient of resistance connected in shunt withthe thermistor. This unit may be proportioned so that it experiences alinear or other change in resistance with temperature. In a. particularsituation, it may be desirable to include a positive reactance elementin series with the thermistor or the shunt resistance to compensate forcap ac1- tance change in the line with temperature change. In anotherembodiment, the compensating units may be spaced uniformly in a loadedtelephone pair, one to each loading section.

A more complete understanding oi the invention will be derived from thedetailed description that follows, taken in conjunction with theappended drawings, wherein:

Fig. 1 shows a portion of a transmission line having inserted thereincompensator units or networks to compensate or correct for attenuationand impedance variation in the line with temperature variation;

Fig. 2 shows one iorm the compensator unit may assume; k

Figs. 3 and i show other configurations that the compensator unit mayhave;

curves at three irequencies for a cable telephone pair with and withoutcompensation in accordance with this invention;

Figs. 6 and 7 show resistance versus'temperature curves for thethermistor alone, the shunt resistance alone and. the compensator unit,inserted at the center and near the ends of the compensated line; i

Fig. 8 shows resistance and reactance and return loss characteristics ofa telephone pair at 3000 C. P. S. with and without attenuation compensation at the center only of a repeater section;

Fig. 9 shows the eflect on impedance changes in a telephone pair of.inserting compensator units at various distances from the endsci arepeater section;

Fig. 10 shows the eflect on the 0 F. impedance at one voice frequency ofinserting various resistances at various distances from the end of a.section of a telephone pair; and

Fig. 11 shows resistance, reactance and return loss versus temperaturecharacteristics for the line having the attenuation-temperaturecharacteristics of Fig. 5. at three frequencies.

With reference to the drawings, Fig. 1, represents a transmission pathor line it comprising a pair of telephone conductors or wires ii of theso-called open wire type, or one pair of many pairs enclosed in a cable.Such a line will experienee variations in its attenuation and impedancecharacteristics with variations in atmospheric temperature. A pluralityof com pensator units or networks i2 is inserted in the line, forexample, in one conductor thereof, at spaced intervals. For reasonssymmetry, noise and cross-talk contrgl, correspondingly located units ornetworks may be inserted in the other conductor also. One network islocated substantially centrally in a repeater section, 1. e., betweenrepeater or amplifier points in the transmission path, and each of twoother networks is located between the center unit and the end of arepeater section but nearer to the repeater point than to the centerunit. In a repeater section of the order of twenty miles in length, theouter units may be of the order of two miles from the ends of therepeater section. The outer units compensate or correct for bothattenuation and impedance changes in the characteristics of the linewith temperature change thereof, and the center unit provides primarilysupplemental compensation or correction for attenuation changes withtcmperature change. In a telephone pair that includes loading coils atspaced intervals, the compensator units may be located at uniformintervals, one to each loading section.

Fig. 2 shows one configuration that the network it may assume. Itcomprises a temperature-dependent resistance element or thermistor T,for example, of a substance or material having a negative temperaturecoefllcient of resistance, with a resistance R- of low or zerotemperature coefficient of resistance connected in shunt therewith. Theresistance T may consist of zinc oxide; silver sulphide; a mixture ofthe oxides of manganese, nickel and cobalt such as is disclosed in F.Dearborn United States Patent 2,274,592 issued February 24, 1942; or amixture of the oxides of manganese, nickel and copper such as isdisclosed in E. F. Dearborn-G. L. Pearson United States Patent 2,282,944issued May 12, 1942. Such material, over the range of atmospherictemperatures, have a substantially exponential resistance-temperaturecharacteristic.

The shunting resistance R. may be of a metal such as copper and isselected with respect to the particular thermistor T so that the twocombined have a resistance-temperature characteristic decreasingsubstantially linearly with increase in temperature over the range ofatmospheric temperatures to which the transmission line is expected tobe subjected. In particular situations, the character of thecompensation or correction desired may make advantageous the inclusionof one or more reactive elements in the compensator unit. Two suchnetworks are shown in Figs. 3 and 4, that of Fig. 3 including a positivereactance element or inductance L in series with the thermistor, andthat oi. Fig. 4 including the inductance L in series with the resistanceRs. With the network of Fig. 3, as the resistance of the thermistordecreases, the element L introduces an increasing inductive component inthe line to compensate for the change in capacitance of the line withtemperature, The

attenuation compensation or correction at the higher frequencies ismaterially improved with the network of Fig. 4 inserted in 'a lineintended to transmit electric waves of frequencies up to, say, 20kilocycles per second. Additional compensator networks or unitsembodying one or more react-ance elements are disclosed and discussed inthe copending application of P. G. Edwards, Serial No. 410,149, filedSeptember 9, 1941, and still others in the copending a lication or R. K.Bullington, Serial No. 410,136, filed Septembe:- 9, 1941.

A better understanding of the invention may be obtained in the light ofthe following specific embodiment. Consider the following situation. Itis desired to compensate or correct substantially for the attenuationand impedance change with temperature change in a repeater section of atransmission line approximately twenty miles in. length and comprising atelephone pair of 19-gauge, .062 microfarad per mile, 1.117 mini--henries per mile, copper wire. The attenuation (in decibels per mile)versus temperature (in degrees Fahrenheit) characteristic of such a lineat three typical frequencies (200, 1000 and 3000 cycles per second) isshown by the curves A, B, C of Fig. 5. The curves A1, B1, C1 and A2, B2,02 show the effect on the attenuation-temperature characteristic of theline of inserting compensator networks therein in accordance withinvention. Each network would be of the configuration shown in Fig. 2and comprise a thermistor of nickel-manganese-cobalt oxides and a shuntresistance of say, copper. The unit or network insorted at the center ofthe line is proportioned to have a resistance-temperature characteristicsuch that it decreases in resistance substantially linearly .from about500 ohms to about 200 ohms over the temperature range of 0' F. to F.Each unit inserted approximately two miles from the end of the linesection is proportioned to have a resistance-temperature characteristicsuch that it decreases in resistance substantially linearly from aboutohms to about 55 ohms over the same temperature range. Curves A1, B1 andC1 show the attenuation-temperature characteristic for the repeatersection when the compensator units are inserted only near the ends ofthe section; curves A2, B2 and C2 show the supplemental attenuationcompensation effect of adding the center compensator unit, resulting ina substantially flat attenuation-temperature characteristic at the threetypical frequencies.

The resistance versus temperature characteristics of the thermistor andof the shunt resistance of the center unit are shown in Fig. 6 as is thecenter unit's resistance versus temperature characteristic; and similarcurves for the end compensator units are shown in Fig. 7.

Fig. 8 shows the resistance and reactance of the transmission line at3000 cycles per second with and without attenuation compensation at thecenter only of a repeater section. The characteristics for return lossversus 55 F. impedance of the line are also shown. The compensator unitwas similar to that specified hereinabove for the center unit in theillustrative example. As indicated by Fig. 8 a single compensator unitat the center of the section does not compensate for changes inimpedance. The effect on impedance changes of providing compensatorunits at various distances from the ends of the repeater section isshown by the curves of Fig. 9. That figure shows the results of ageneralized computation of the required impedance Az at a miles Torecapitulate in part and also to aflord a further understanding of ourinvention, the followving statements may be made. In a non-loadedtelephone pair, if a series impedance is added at F. without the seriesimpedance. It can be shown that where Znm =characteristic impedance ofcable at temperaturet. t =some temperature less than 110 F. at

which it is desired to equate impedance to that of 110 F. (t=) whenp.=0, the equation reduces to Az =Zo(110)-Zo(t) Fig. 9 indicates that aresistance of the order of 65 ohms at a distance of two miles from theterminal of the repeater section will make the 3000 cycles per secondinput impedance at 0 F. equal that at 110 F. without added resistance.This does not perfectly compensate for other frequencies. To maintainthe compensator unit of simple character, 1. e., to employonlyresistance changes, the effect on the change in impedance of addingvarious resistances at variousdistances. bearing in mind the importanceof the 3000 cycles per second frequency from a voice frequenc repeaterstandpoint, should be considered. It can be shown that 1 tanh P sistentwith the desired results as, for example,

P =propagation of the telephone pair The relation of Equation 2 holds atany temperature assuming that P and Z0 are for that temperature, andcan-be applied, also, to determine the efiect of nearby resistances forimpedance compensation, or of attenuation compensator units at thecenter of the section. Fig. 10 shows the modification of the 0 F.impedance at 3000 cycles per second caused by the insertion of 60, 70and 80 ohms at 0, l, 2, or 3' miles. The effect changes from pureresistance change at zero mile to substantially pure reactance change atthree miles.

The curves of Fig. 11 show the resistance, reactance, and return lossversus temperature characteristics for the line having theattenuationtemperature characteristics of Fig. 5, at 200, 1000 and 3000cycles per second. The resistance and reactance versus temperaturecharacteristics are seen to be substantially flat over the temperaturerange of 0-ll0 I.

the terminal, it will modify the terminal impedance by algebraicaddition of the resistance and reactance components. As the added seriesimpedance is moved farther away from the terminal, the modifying effectbecomes less because of the shunting due to the intervening cable, sothat at a distance of, say, 6 to 10 miles, the modification of theterminal impedance is much smaller. The change in attenuationisapproximatelythe same for a given added series impedance regardless ofthelocation in the line. An

arrangement is feasible, then, where added impedance near the end of thecable modify both impedance. and attenuation, and added impedances nearthe middle of the cable modify atten-- uation primarily, and bothimpedance and attenuation changes with temperature normally occuring maybe substantially canceled or compensated for by the added i'mpedances.

A study of the impedance required to cause a desired change in theoriginal terminal impedance shows that for various distances of forexample 0, 1, 2, 3 etc. miles out from the terminal impedances, addedseries impedances Z0, Z1, Z2. Z3, etc. can be found which will cause thedesired modification at the terminal. At certain critical distancestheseimpedances are resistances, even though the resulting terminalchanges are reactive in character. The ideal impedance and impedancechanges are of course different for different frequencies.

Since the same added impedance applies to all frequencies, attention iscentered on the most critical frequency or frequencies, and the effectof the optimum impedance for this frequency or frequencies is determinedfor other frequencies of interest. Also, it is desirable to keep theadded impedance to a minimum of complexity conpure resistance only.

If for a given frequency, say 3000 cycles, investigations are made ofthe effect of various amounts of resistance added in series with anonloaded telephone pair at various distances, it will be found that apure resistance of about 65 ohms at a distance of 2 miles from theterminal will cause the required impedance change to compensate for thechange in impedance of the line at 3000 cycles, 0 F. to F'., and thatthe re- .quired change and theresulting compensation are substantiallyproportional, so that, if it is assumed that the added resistance is 0ohms at 110 F. and 65 ohms at 0 F., and changes proportionally forintermediate temperatures, satisfactory 3000 cycle compensation willresult.

At other frequencies, it will be found that the same resistance requiredto compensate 3000 cycle temperature impedance changes will provide agood degree of compensation at 1000 cycles and slightly inferior degreeof compensation at 200 cycles. provide separate units for a betterdegree of compensation at all frequencies, or a more complex unitinvolving reactances which would match the required compensation changesfor several frepedance will be higher than for an idea compen-.

sating resistance.

It would, of course, be possible to Now, if the attenuation changesproduced by a compensator unit near each terminal are determined, itwill be found that insumcient attenuation compensation is provided, andthe additional attenuation compensation can be added at or near themiddle of the cable section without seriously modifying the impedancecompensation already obtained. In fact, it i possible to arrive at adesign in which the effect of the center compensation on the terminalimpedance is taken into account. The general procedure is to addalgebraically the impedance compensation caused by the terminal andcentral units.

In the case of a loaded telephone pair, as in- I dicated in thecopending application noted above in the name of P. G. Edwards, SerialNo. 410,149, the locationin the loading section, the frequency withwhich the compensating units are provided, and 'the accuracy of spacingare all important considerations. The compensating units should beregularly spaced, and one loading section or a multiple thereof apart.End sections may or may not be compensated depending on their length.Half sections, for example, might have half compensation.

Where the location in the section is unsymmetrical, as for example, atthe0.2-0.8 point, and this spacing is preserved throughout the repeatersection, the impedances' as seen from the two ends would be somewhatdifferent at low frequencies.

. This conditioncan be obviated by making the two halves of the linealike. For example, with half section termination, it may be desirableto place a half section compensator in the end section and 0.2 from thefirst loading coil, with subsequent compensators at intervals of oneloading section up to the center of the repeater section. If this isdone from either end, generally speaking the terminal compensatedimpedances will be alike since the effect on the terminal impedancediminishes as the distance of the compensator unit from the terminalincreases. At the center, one of two conditions will prevail: for aneven number of sections the two compensators near the center will be 0.4section apart; for an odd number of sections the center compensator maybe placed in the center of the center section.

The intervals at which the compensator units are provided is governed bythe smoothness required in the line impedance. For one compensator perloading section, the impedance is smooth throughout the range to cutoff.For one compensator per two loading sections, there is an impedanceirregularity at 0.7 of the cutoff frequency, and so on, the 'fewer thecompensator units the greater the impedance irregularity with more andmore of the useful frequency range affected.

The eflect of varying the location of the compensator units from thetheoretical location is V like that caused by varying the location ofloading coils from the theoretical in that it reduces the return losswhich can be obtained because of impedance irregularities, but theeffect is not as severe since at most frequencies resistance has littleeflect on impedance. At low frequency, where the effect is appreciable,the line tends to be transparent and the'location is not critical.

Because of the modification of the low frequency impedance by thecompensator units, it is advantageous to modify the balancing networksemployed to balance the compensated lines in connection with the use oftelephone the regular network a compensating series element made up ofresistance and capacitance in shuntto cause ,the same change in thenetwork impedance from say 200 cycles to 1000 cycles as the linecompensator units cause in the line impedance.

While this invention has been disclosed as embodied in certain specificforms which are deemed desirable, it is understood that it is capable ofembodiment in many andother wide- ,ly varied forms without departingfrom the spirit of the invention as defined by the appende claims.

What is claimed is:

1. In combination, a transmission line that varies in attenuation andimpedance with variations in temperature, and means inserted in asection of said line to compensate for said variations in said section,said means comprising an impedance unit nearer each end of the sectionthan thecenter thereof, and an impedance unit at substantially thecenter of said section com: pensating primarily for attenuationvariation in said section, each of said units being subjected to thesame temperature variation as said line and including a resistanceelement that varies in resistance in a direction opposite to thevarirepeaters. A simple way to do this is to add to ation in resistanceof said line with temperature variation.

2. In combination, a transmission line that varies in attenuation andimpedance with variations in temperature, a pair of repeaters in saidline at spaced intervals along said line, and means inserted in saidline intermediate said repeaters to compensate for said variations, saidmeans comprising three impedance units variable in impedance withtemperature variation and subject to the same temperature variation asthe line, one of said units being located at a point in said lineapproximately equally distant from each of said repeaters, and each ofthe other units being located at a point in the line unequally distantfrom the two repeaters.

3. The combination of claim 2 in which said one unit compensatesprimarly for attenuation change in the line betweerithe repeaters, andsaid other two units compensate substantially entirely for the impedancevariation in the line between said repeaters.

4. In combination, a transmission line that varies in attenuation andimpedance with temperature variations and being of a material having apositive temperature coefllcient of resistance, impedance means insertedin said line to compensate for said impedance change, and additionalimpedance means inserted at another point in said line to compensateprimarily for attenuation change in said line, said impedance meansbeing subjected to the same temperature variations as said line and eachincluding a resistance element having a negative temperature cocflicientof resistance.

5. In combination, a transmission path comprising a section of atransmission line that varies in attenuation and impedance withvariation in temperature, a substantially pure resistance inserted insaid section nearer to one end than to the center. thereof to providecompensation for said impedance variation, and an additionalsubstantially pure resistance inserted at another point in said sectionto provide compensation'for said attenuation variation, each of saidresistances being variable" in resistance with temperature variation andsubject to the same temperature variation as theline.

6. In combination, a transmission path comprising a section of atransmission line that varies in attenuation and impedance withvariation in temperature, a substantially pure resistance inserted insaid section to provide compensation for said impedance variation. andanother substantially pure resistance inserted at another point in saidsection to provide compensation for said attenuation variation. each ofsaid resistances being variable in resistance with temperature variationand subject to the same temperature variation as the line.

7. In combination, a transmission path comprising a section oftransmission line that varies in attenuation and impedance withtemperature,

and a plurality of attenuation and impedance variation compensator unitsinsertedin said section and being variable in impedance with temperatureand subject to the same temperature variation as the line. one of saidunits being located in the central portion of said section and anotherunit being located nearer to one end of said section than to the centerthereof, the centrally located unit being of a diflerent resistance overthe expected temperature range than the end unit.

8. The combination of claim 7 in which said centrally located unit is ofa higher resistance over the expected temperature range than the endunit. V

' 9; The combination 0! claim 7 in which said centrally located unitprovides compensation primarily for attenuation variations and the endunit provides compensation for attenuation and impedance variations.

LEONARD G. ABRAHAM. PAUL G. EDWARDS. RENATO FRACASSI.

